After signing an executive order enforcing tougher sanctions on Russia, President Obama today denounced a planned referendum to split Crimea from Ukraine and called for international monitors in the tense region to ensure citizens’ rights are being respected.

Talking tough on an international crisis as serious as any conflict since the Cold War, Obama stressed his preference for diplomacy even as he outlined measures to punish the Russian government for violating international law with its military incursion into Ukraine.

“If this violation of international law continues, the resolve of the US and the international community will remain firm,” Obama said at a White House briefing.

“We have taken steps to reaffirm our commitment to the security and democracy of our allies in Eastern Europe.”

Those measures included visa restrictions and economic sanctions on Russians involved in the decision to move the military toward action in Ukraine.

The new steps also block financial transactions of designated Russians.

With his executive order Obama rejected the pro-Moscow Crimean administration’s referendum on the area’s future, however, saying it would violate Ukraine’s constitution.

“The proposed referendum on the future of Crimea would violate the constitution and violate international law,” Obama said. “We are well beyond the days when borders can be redrawn over the heads of democratic leaders.”

Many in Ukraine fear Crimea’s referendum move is a pretext toward secession and the eventual annexation by the Russian Federation.